PROJECT SUMMARY (Project 8; Maier, Chorover, Neilson, Barton) Mine tailings are wastes leftover following the crushing of ores and extraction of metals such as copper during the mining process. This waste is devoid of the properties that make soils a suitable habitat for plants including soil structure, organic matter, nutrients and microorganisms. These tailings deposits, which range from several to thousands of acres in size, can remain barren for decades or longer resulting in wind and water erosion into the surrounding environment. Of particular concern are legacy mine tailings which are often abandoned and are characteristically highly acidic with elevated levels of toxic metals such as arsenic and lead. Often these metals are associated with very small tailings particles that are easily windborne. Thus, these tailings have the potential to impact both human and environmental health in areas surrounding such waste sites. The overall goal of this project is to understand, refine, and develop industry and regulatory guidance for an interventional remediation technology known as phytostabilization. Phytostabilization involves the establishment of a vegetative cap on mine tailings using plants that do not accumulate metals into their shoot tissues. Instead, the metals are sequestered in the root zone resulting in decreased mobility either by wind or water. Our research to date also suggests that phytostabilization can act to change the form of metals in mine tailings thereby reducing their toxicity. The specific aims of this project are to (1) continue a long-term field trial of compost- assisted phytostabilization of the mine tailings at the Iron King Mine and Humboldt Smelter Superfund site to understand the longer-term outcomes (years 5-10) of this technology; (2) evaluate the specific interactions among roots, microorganisms and metals that drive the success (or failure) of plants growing in mine tailings; and (3) use the knowledge gained and a partnership that has been established with the mining industry to expand our research to additional sites to identify universal indicators of phytostabilization success. These indicators will be the basis for development of a guidance on phytostabilization that can be used by both the mining industry and regulators to manage abandoned mine tailings sites as well as sites that are slated for closure.